1. Field of Invention
The current invention relates to porous metals, and more particularly to porous metal catalysts for oxygen reduction, methods of producing the porous metals and devices that use the porous metals.
2. Discussion of Related Art
All low-temperature proton exchange membrane fuel cells (PEMFCs), such as are envisioned for automotive applications to replace the internal combustion engines, require two catalytic electrodes: one to extract protons and electrons from fuels such as hydrogen or methanol, and one to re-combine these protons and electrons with oxygen to form water. Regardless of the fuel used in low-temperature polymer electrolyte membrane fuel cells (PEMFCs), be it hydrogen (J. Erlebacher, Solid State Physics 61, 77 (2009)), methanol (H. Liu, et al., J. Power Sources 155, 95 (2006)), or ethanol (A. Kowal, et al., Nature Materials 8, 325 (2009)), the primary catalytic bottleneck to the use of these devices are the slow kinetics of the cathodic oxygen reduction reaction (ORR) in which an oxygen molecule is reduced to water via a complex reaction pathway involving four electrons and four protons. Sluggish ORR kinetics accounts for approximately 80% of the losses in PEMFCs (T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146, 3750 (1999)). The most widely used and studied catalyst for the ORR has been Pt, but even on this single-component material the detailed reaction mechanism remains controversial as oxygen reduction on Pt is sensitive to many factors including catalyst crystal surface orientation (N. Markovic, H. Gasteiger, P. Ross, J. Phys. Chem. 99, 3411 (1995); C. Zinola, A. Luna, W. Triaca, A. Arvia, Electrochim. Acta 39, 1627 (1994)), whether the catalyst form factor is nanoparticulate or bulk metal (E. Higuchi, H. Uchida, M. Watanabe, J. Electroanal. Chem. 583, 69 (2006)), and the electrolyte anion species (J. Wang, N. Markovic, R. Adzic, J. Phys. Chem. B 108, 4127 (2004)). There are two approaches being pursued to enhance the ORR for fuel cells: (1) the development of catalysts that exhibit only moderate activity when compared to Pt, yet are inexpensive and can be produced in significant quantities (M. Lefevre, E. Proietti, F. Jaouen, J. Dodelet, Science 324, 71 (2009)), and (2) developing more Pt-based nanostructured alloy catalysts that, while still perhaps expensive, potentially yield orders of magnitude higher activities than Pt alone via a variety of mechanisms such as changes in the electronic structure, e.g., shifts in the d-band center, leading to more favorable interactions with reactants and products (V. Stamenkovic, et al., Science 315, 493 (2007); R. R. Adzic, et al. Top. Catal. 46, 249 (2007); B. Hammer, J. K. Norskov, Adv. in Catalysis 45, 71 (2000)).